The invention relates to a power steering device for a vehicle.
Such a power steering device, which is employed as a servo steering assistance system in motor vehicles, is described in EP 0 397 206 B1. The power steering device comprises a servo cylinder having two pressure chambers, which are separated by a working piston and supplied with hydraulic fluid by a pressure supply unit. Depending on the desired working movement of the piston, either one or the other pressure chamber in the servo cylinder is subjected to pressurized hydraulic fluid.
In order to prevent hard impact of the working piston against the limit stops in the servo cylinder in the region of maximum travel, the power steering device is equipped with a pressure relief unit, which opens a bypass between the two pressure chambers as the working piston reaches the axial end position, whereupon pressure equalization takes place between the pressure chambers so that further travel of the working piston in the direction of the limit stop is blocked. The pressure relief unit comprises a valve plunger, which is held movably on the working piston and during normal operation seals the bypass between the pressure chambers.
Upon approaching the limit stop, the movable valve plunger moves against an adjustment bushing fixedly disposed in the servo cylinder and is adjusted by said bushing from the sealing position into an open position, thereby opening the bypass and enabling the transfer of hydraulic fluid between the pressure chambers. This pressure limitation upon reaching the limit stop has the effect that the pump is not forced to work against the pressure control valve and thus counter to the maximum operating pressure, which in addition to noise would also result in strain on the pump due to overheating. The limit stop is additionally protected from mechanical overload and/or deformation. Finally, a drop in the motor speed of the pressure pump is prevented.